We have identified an outbred colony of Mus cervicolor popaeus established from a feral population in Asia which has a high incidence of mammary tumors. Other colonies of M. cervicolor popaeus derived from mice caught in other locations or other species of Mus from the same general location have a low or no incidence of mammary tumors. Most of the mammary tumors histologically fall into adenocarcinomas type A, B or C; however, novel types of tumors are also observed including ductal carcinomas. In many but not all of the tumors type B retroviral particles were observed. Interspecies radioimmunoassays for MTV - associated proteins showed that the tumor-associated virus is antigenically indistinguishable from the M. cervicolor milk-borne type B retrovirus MC-MTV which has previously been shown by us to be distantly related to the laboratory strains of MMTV from M. musculus.